1. Field of the Invention
The present invention relates to a received light level indicating system for a beam sensor.
2. Description of the Related Art
One known type of system for detecting an intruder includes a beam sensor system in which a light projector for projecting an infrared beam (hereinafter referred to simply as "beam") and a light receiver for receiving the beam from the projector are disposed opposite each other. In the beam sensor system, the presence of an intruder is detected on the basis of the quantity of infrared radiation entering the light receiver.
Accordingly, the beam sensor system requires an optical axis adjusting operation in order to make the projecting optical system of the light projector and the receiving optical system of the light receiver face each other accurately. In the optical axis adjustment, the degree of accuracy of alignment between the optical axes of the light projector and the light receiver has heretofore been judged by using a tester. In general, the light receiver is provided with a terminal for outputting the level of received light, e.g., the peak voltage of the received beam, and a tester is connected to the terminal to read the level of the received light. With the conventional practice, a higher voltage level shown by the tester reveals a higher degree of accuracy of alignment made between the two optical axes.
Accordingly, it has heretofore been common practice to effect optical axis adjustment by changing the angles of the projecting and receiving optical systems in the vertical and horizontal directions so that the received light level measured with the tester at the light receiver side reaches a maximum.
However, the conventional practice is considerably troublesome because the tester must always be carried to the job site to effect optical axis adjustment. In addition, because the angles of the optical systems of the beam sensor are changed in the vertical and horizontal directions to effect optical axis adjustment, the received light level changes, and the pointer of the tester moves in a corresponding manner. Since the movement of the pointer of the tester is continuous, it is difficult to judge the position of the maximum received light level and the direction of change of the received light level, that is, whether the received light level is increasing or decreasing.
To cope with these problems, a system has recently been proposed. In the proposed system, a level meter comprising an array of a plurality of light-emitting elements, e.g., LEDs, is provided, and the number of light-emitting elements turning on is controlled in accordance with the received light level. With the proposed system, it is unnecessary to use a tester for each optical axis adjusting operation as in the conventional practice, and it is possible to judge the received light level by the number of light-emitting elements turning on in the level meter. Further, the direction of change of the received light level can surely be understood from the change in the number of light-emitting elements turning on.
However, such a level meter needs a plurality of light-emitting elements and requires a larger number of light-emitting elements in order to indicate the received light level even more finely. Accordingly, the cost rises, and the power consumption increases.